Let's say you go with 3GB for $30 and only use 1.4GB one month. You'll get $16 back, so you only pay for what you use.

Simple.

It's a great deal. T-Mobile and AT&T let you roll over unused data to the next month, but you still have to pay the full price for that data regardless of whether you end up using it. AT&T and Verizon also charge you extra if you go over your data plan. Google is the only company willing to refund you in cash for the data you don't use.

Now for the caveats.

Project Fi is available only in limited areas in the US, and it works only on the Nexus 6 phone, which is made by Motorola. There aren't any immediate plans to open Project Fi to other devices like the iPhone quite yet. Google is also using Sprint and T-Mobile's networks to host the data, but Sprint and T-Mobile generally have poorer coverage compared with AT&T and Verizon.

That said, it's a step in the right direction. And if Project Fi or something like it grows big enough, it could entice big carriers like AT&T and Verizon to offer more-consumer-friendly pricing plans, too.